Apparatus for cooling steel strips (this term includes plates too) to effect continuous annealing thereof are classified into two types, one so-called gas jet type and the other water cool type. Although each type has an inherent advantage, it accompanies certain defects.
More particularly, according to the gas jet system cooling gas is blasted at a high speed against a steel strip heated to a temperature of about A.sub.1 transition point to cool the strip to about 400.degree. C. and then the strip is subjected to a super aging treatment for 3 to 5 minutes. The cooling efficiency of gas is inferior than that of liquid because of its small thermal capacity. Although liquid has a larger thermal capacity than gas, vapour film formed on the surface of the strip decreases the cooling efficiency. For this reason, in a modern high speed large processing line, the cooling equipment is large, expensives and requires a high running cost.
However, the gas jet type has an advantage that its cooling speed can be adjusted at will which is suitable for soft steel strip, and the heat cycle of this type is economical because in contrast to the water cooling type it is not necessary to first cool the coolant to room temperature and then heat it to a super aging temperature.
Among the water cooling types, i.e., water quenching systems are included a method in which a liquid coolant is ejected upon a uniformly heated steel strip and a method in which a heated steel strip is dipped in a liquid coolant. To eject liquid, not only is special ejection equipment necessary but also the pattern of the ejected cooling liquid varies, thus failing to obtain uniform cooling and homogeneous product.
The heat cycle of each cooling system is fast. Especially, with the dip method since the cooling speed is especially high, i.e., of the order of 1000.degree.-2000.degree. C./sec., which should be compared with 10.degree.-30.degree. C./sec. of the gas jet method, the dip method is suitable for manufacturing high tension steel stocks having a mixed structure of ferrite and martensite and not containing any other special elements. As above pointed out since this method ensures a high speed cooling, the cooling equipment for a high speed steel strip processing line is extremely compact. Moreover, as it is sufficient to merely pass the steel strip through cooling water the running cost can be greatly reduced.
In spite of the advantages described above since the cooling speed is too rapid, even when the cooling water is heated to the boiling point thereof, it is impossible to transfer to the super aging temperature during cooling and since the steel strip subjected to cooling becomes about 100.degree. C. or normal temperature, where super aging treatment is performed subsequent to quenching it is necessary to reheat the steel strip which has been cooled to such a low temperature which requires additional process steps and equipment resulting in a poor thermal efficiency.